Finding reapited data in a table

Hi I have set of ISSN numbers about 20,000 of them in a table which some are repeted hence I cant make this field as a primary key, is there a simple sql command i can use to find these repeated numbers and output them (without deleting them first because i need to keep a record of it)...

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Defining the name:
When I talk to people they say different names on this subject stuff l…